1. Field of the Invention
The present invention relates to a fixing unit for an image forming apparatus.
2. Description of the Related Art
An image forming apparatus such as an electronic photo printer usually forms an electrostatic latent image on a photosensitive drum using a laser beam or the like, and forms a toner image by developing the electrostatic latent image with a toner. Then, it transfers the toner image onto printing paper, fixes the toner image on the printing paper with a fixing device having a fixing roller, and then ejects the paper. Such electronic photo printer and fixing device are disclosed, for example, in Unexamined Japan Patent Application Publication 07-181833 and Unexamined Japan Patent Application Publication 2001-242741.
FIG. 15 schematically illustrates the constitution of the electronic photo printer disclosed in Unexamined Japan Patent Application Publication 07-181833. This electronic photo printer is comprised of a paper cassette 1 to store printing paper 20, a paper feeding roller 2 to feed the printing paper, a paper conveyance roller 4 to convey the printing paper 20, a photosensitive drum 5 to transfer a printing toner (not illustrated) onto the printing paper 20, a fixing device to fix the transferred toner on the printing paper 20, and a stacker 22 to store the paper 20 after printing.
This electronic photo printer further comprises a cassette sensor 25 to detect if the paper cassette 1 is inserted and in a position, and a paper feeding sensor 3a which is provided between the paper feeding roller 2 and the paper conveyance roller 4 to detect the presence of the printing paper 20. In addition, a charger 6 to uniformly charge the surface of the photosensitive drum 5, an LED head 7 to form an electrostatic latent image that corresponds to the shapes of characters and the like by irradiating a specified light beam onto the surface of the charged photosensitive drum 5, a developing device 8 to form a toner image by developing the electrostatic latent image through adhering toner on the electrostatic latent image, a transferring device 9 to transfer the toner image onto printing paper 20, and a cleaner 10 to remove the toner remained on the photosensitive drum 8 are provided around the photosensitive drum 5.
Furthermore, the fixing device 21 has a heater 21a provided inside the roller, and a thermistor 21b to detect temperature change of the roller. By heating and pressuring with the roller the toner image transferred on the printing paper 20, the toner image is fixed. A paper ejection sensor 3b to detect the presence of the printing paper 20 passing through the fixing device 21 is provided between the fixing device 21 and the stacker 22.
Moreover, the electronic photo printer has a first controlling unit 23a to control the operation of the mechanism of each element, an operation panel 24 to notify an operator of specified alarm and to set a menu, and a second controlling unit 23b to control the operation panel 24, communicate with upper device(s), edit printing data, and so on. The first controlling unit 23a and the second controlling unit 23b are connected to each other via a serial interface and a video interface.
FIG. 16 is a circuit diagram of the first controlling unit 23a which controls the operation of the mechanism of the above-described electronic photo printer. A microprocessor 13c of the first controlling unit 23a mainly outputs a motor 1 control signal (a), a motor 2 control signal (b), a motor 3 control signal (c) and a heater control signal (h) based on a comparator output signal (g), a paper feeding sensor signal (d), a paper ejecting sensor signal (e), and a cassette sensor signal (f).
Here, the motor 1 control signal (a) is a control signal for the motor 1, which is outputted from an output port P3.0 of a microprocessor 13c, and drives the photosensitive drum 5 and the roller of the fixing device 21 shown in FIG. 15 via the motor controlling circuit 14. The motor 2 control signal (b) is a control signal for a motor 2, which is outputted from an output port P3.1 of the microprocessor 13c, and drives the paper feeding roller 2 shown in FIG. 15 via the motor control circuit 14. The motor 3 control signal (c) is a control signal for a motor 3, which is outputted from an output port P3.2 of the microprocessor 13c, and drives the conveyance roller 4 shown in FIG. 15 via the motor control circuit 14. The motor control circuit 14 is connected to an oscillator 15. The rotational speeds of the motors 1-3 are determined by the values of frequency oscillated by the oscillator 15.
The paper feeding sensor signal (d) is an output signal from the paper feeding sensor 3a shown in FIG. 15, and is connected to an input port P2.1 of the microprocessor 13c. The paper ejecting sensor signal (e) is an output signal from the paper ejecting sensor 3b shown in FIG. 15, and is connected to an input port P2.2 of the microprocessor 13c. The cassette sensor signal (f) is an output signal from the cassette sensor 25, and connected to an input port P2.3.
A comparator output signal (g) is a signal outputted from a comparator 12, and is connected to an input port P2.0 of the microprocessor 13c. The input terminal a of the comparator 12, to which the reference voltage is applied, is connected to a pull-up resistance R3, a pull-down resistance R4, and a hysteresis resistance R2. The input terminal b of the comparator 12 is connected to the pull-up resistance R5 and the terminal a of the thermistor 21b, while the terminal b of the thermistor 21b is connected to ground. The output terminal c of the comparator 12 is connected to the pull-up resistance R1, the hysteresis resistance R2, and the input port P2.0 of the microprocessor 13c. 
In addition, the heater control signal (h) is outputted from an output port P1.0 of the microprocessor 13c, and is connected to a power control circuit (not illustrated) of the heater 21a of the fixing device 21 shown in FIG. 15 so as to perform ON/OFF control of the heater 21a. 
The thermistor 21b shown in FIGS. 15 and 16 has the characteristics of having smaller resistance as the temperature becomes higher, and larger resistance as the temperature becomes lower. For this reason, the voltage Vb applied to the input terminal b of the comparator 12 becomes lower as the temperature becomes higher, and the voltage Vb becomes higher as the temperature becomes lower.
FIG. 17 is a time chart in the temperature control of the heater 21a using the above-described characteristics. Here, Vb shows the waveform of the voltage applied to the input terminal b of the comparator 12, Va-High is the reference voltage applied as a high slice level to the input terminal a of the comparator 12, and Va-Low is the reference voltage applied as a low slice level to the input terminal a of the comparator 12. TL is the length of time from when Vb reaches Va-High to when Vb reaches Va-Low, and TH is the length of time from when Vb reaches Va-High to when Vb reaches Va-Low.
Once the voltage Vb applied to the input terminal b of the comparator 12 exceeds the high slice level voltage applied to the input terminal a of the comparator 12, the comparator output signal (g) becomes low level. The microprocessor 13c shown in FIG. 16 detects the comparator output signal (g) which is low level at the input port P2.0, and turns “ON” the heater 21a, setting the output port P1.0 as high level.
When the heater 21a is turned “ON”, the temperature of the roller of the fixing device 21 becomes higher, the resistance of the thermistor 21b becomes smaller, and the voltage Vb applied to the input terminal b of the comparator 12 becomes lower. Once Vb becomes lower than the low slice level Va-Low applied to the input terminal a of the comparator 12, the comparator output signal (g) becomes High level. The microprocessor 13c detects the comparator output signal (g) which becomes high level at the input port P2.0, and turns “OFF” the heater 21a, setting the output port P1.0 as low level. By repeating the above-described control of the heater 21a from when the power of the electronic photo printer is ON till the power is OFF, the temperature of the roller of the fixing device is maintained at a substantially constant temperature.
Referring now to a time chart of FIG. 18, the method of controlling the above-described electronic photo printer is described. Here, the reference numerals not indicated in FIG. 18 shall be referred to those in FIGS. 15 and 16. Once the second controlling unit receives printing data from the upper device (not illustrated), it produces an image data for printing, and then writes the data in a memory (not illustrated). Then, once editing one page of the image data is completed, a printing instruction is sent to the controlling unit 23a. 
Once the first controlling unit 23a receives a start printing instruction from the second controlling unit 23, it rotates the photosensitive drum 5 and the roller of the fixing device 21, setting the motor 1 control signal (a) as high level. At the same time, by driving the charger 6, the surface of the photosensitive drum 5 is uniformly charged. After that, the motor 2 control signal is set as high level, and a sheet of the printing paper 20 in the paper cassette 1 is fed toward the conveyance roller 4. Once the paper feeding signal (d) becomes high level (i.e. if the paper 20 is passing the paper feeding sensor 3a), the rotation of the paper feeding roller 2 stops, setting the motor 2 control signal (b) as low level after a specified length of time (until the front edge of the paper contacts the conveyance roller 4).
Once the rotation of the paper feeding roller 2 is stopped, the paper 20 is fed toward the fixing device 21 by rotating the roller 4, setting the motor 3 control signal (c) as high level. At this time, the second controlling unit 23b sends the image data for printing to the LED head 7 via the first controlling unit 23a, and forms an electrostatic latent image on the surface of the photosensitive drum 5. Then, the toner image is formed from the electrostatic latent image by the developing device 8, and then transferred onto the paper 20. The paper having the transferred toner image is conveyed to the fixing device 21, and the toner image is fixed on the paper 20 by heating and pressuring. Thereafter, the paper that passed the fixing device 21 reaches the paper ejecting sensor 3b, and the paper ejecting sensor signal (e) becomes high level. Then, if the paper is further conveyed, and the paper ejecting sensor signal (e) becomes low level, the paper 20 after printing is stored in the stacker, and the printing is completed.
After completing the printing, the first controlling unit 23a notifies the completion of the printing to the second controlling unit 23b. Once the second controlling unit 23b receives the notification of the completion of the printing, it clears the memory, in which a page of image is written. If there is no printing instruction from the second controlling unit 23b within a specified period of time after the paper 20 passes the paper ejecting sensor 3b, the first controlling unit 23a stops the rotation of the photosensitive drum 5 and the fixing device 21. On the other hand, if there is a printing instruction from the second controlling unit 23b within the specified period of time, the paper feeding roller 2 is rotated, setting the motor 2 control signal (b) as high level, and printing on the next sheet of the paper 20 is performed. By repeating the above-described operations, a series of printing process is performed.
As described above, in the image forming device such as the electronic photo printer, the temperature of the roller of the fixing device can be detected without contacting the roller surface. Therefore, the thermistor is attached to the frame being specified distance away from the surface of the fixing roller, and the timing for turning ON/OFF the heater in the fixing roller is controlled based on the temperature detected by the thermistor. By heat of the heating element and pressure applied by the fixing roller, the toner is fixed on the printing paper, and then the printing is completed.
However, since the distance between the fixing roller and the thermistor varies depending on the dimensional precision or precision of attachments related to a frame or other components, the temperature detected by the thermistor of conventional fixing device varies, which affects the temperature control of the roller surface of the fixing device.